Diagnostic Imaging of Congenital Heart Disease.
Hans G Ringertz, MD, PhD
Department of Radiology and Paediatric Radiology, Karolinska Hospital, Stockholm, Sweden.
(Радиология в медицинской диагностике [современные технологии] 2003: 74-77)

There are a number of imaging options available for the evaluation of the anatomy of the heart and congenital heart disease. They have different values under different circumstances and the continuous technical development tends to change these values. In reality combinations are used and they used to depend more on the local availability and experience than on hard scientific facts or evidence-based radiology. The different options available in imaging in these respects are seen in the table below. The relative merits of these methods suggested vary with the anatomical structures involved and the clinical situation. It can be stated that the usefulness of MRI from the point of view of global understanding of the patho-anatomical and physiological situation in a congenital heart malformation is good and in my estimate increasing due to the technical development

Technical aspects.
There are two principally different frequently used magnetic resonance imaging techniques for congenital cardiovascular abnormalities. The first one is T1 weighted spin echo sequences in different planes that are used to image the anatomy of the heart and central vascular structures. The second is the faster gradient echo sequences primarily used to image functional aspects of the cardiovascular system. Such an examination can be used for cine-MR of an average cardiac cycle as well as phase contrast flow measurements. Using gradient echo the flowing blood has high signal intensity - white - but with spin echo technique it has low signal - black.

The basis for useful imaging of the heart and great vessels with MRI spin echo sequences is as follows: The contrast between absence of signal from hydrogen nuclei in rapidly flowing blood, the low signal from the lung tissue, and the intermediate signal from the walls of the cardiac chambers and vessels. From the imaging point of view there is some loss of signal due to cardiac motion. This hampering fact increases when imaging patients with rapid or especially irregular heart rates.

In order to image infants or children with congenital cardiovascular abnormality ECG triggering via non-magnetic transmission of the R-wave impulse is used. This normally starts a spin-echo MR sequence at varying time after the R-wave producing images at any phase of the cardiac cycle. The repetition time is dependent on the time between two consecutive R-waves. The echo time and pulse frequency together determine the number of slices that can be evaluated. Fast heart rate results in shorter examination time but also fewer anatomical slices.

MRI can obtain images in any axial, coronal or sagittal plane. If however planes corresponding to the axes of the heart are required, changed spatial angles can be used. Such oblique projections are very useful in imaging atrial or ventricular septa and the great vessels, specifically aorta and the pulmonary artery.

With spin echo a volume of blood in vessels and cardiac chambers have low or no signal due to the so called ”flow void”. The reason is that fast flowing blood exit the magnetised volume before its echo-signal has been sent out. The speed with which blood has to flow in order to lose signal depends on slice thickness relative to echo time. An other important factor in this respect in 2D MRI imaging is if the flow is within the imaging plane or at an angle to the plane.

Using low flip angles in fast sequences with gradient recalled echoes makes it possible to get phase contrast magnetic resonance images of flowing blood very fast. In this case the blood appears bright and utilising the echo information the flow can with this technique be viewed in a cine loop. From the simultaneous phase information physiological flow measurements can be performed. Applications of both these techniques are routinely used with problems connected with congenital heart lesions.

With gradient echo technique, turbulence dephase the spins and cause decreased intensity of the flowing blood even when flow is slow. This is used for evaluation of stenotic lesions or valvular insufficiency. The phase-contrast technique maximises the contrast between pulmonary vessels and the low signal intensity lung tissue but in most instances the flow-void is sufficient for the study of congenital vascular abnormalities.

Practical aspects.
Neonates, infants, and children up to 5 or 6 years of age are generally sedated. There are many articles and textbooks that provide drug regimens for this sedation of children undergoing magnetic resonance examinations. Besides full anaesthesia, oral chloral hydrate or intravenous pentobarbital in younger children and bensodiazepin derivatives to the anxious older child seems to be most common.

If the child has a cyanotic congenital malformation, vascular compression, or other causes of airway compromise administration of oxygen is recommended, especially to the sedated patient. Each institution for regular use should devise a safe and generally accepted sedation regimen. Both because of the congenital lesion and the sedation the respiration and cardiac status should be monitored. Displaying the ECG in the control room on an oscilloscope is often combined with a cutaneous oximeter to continuously monitor oxygen saturation. Transmitters on the chest can also detect the respiration remotely and this practice can be and is in our instance routinely utilised.

There are few contraindications to magnetic resonance imaging of congenital heart disease. Pacemakers are however an absolute such contraindication to exposure both to static and gradient magnetic fields. Because parents or other adults frequently accompany children into the scanning room they should also be controlled in this respect.

There are also concerns regarding the safety of magnetic resonance imaging of patients with surgical implants, vascular clips, and prosthetic heart valves. Sternal wires produce an area of signal void, but it does not normally affect the visualisation of the anterior part of the heart. Most metallic objects used in these areas today are adapted not to impose a danger to the patient when examined with magnetic resonance imaging but they do often produce image artefacts.

Clinical applications.
Magnetic resonance imaging makes it possible to study most anatomical details of the paediatric heart. The wide field of view compared to ultrasound, makes it possible to assess the relative size and positions of the cardiac chambers as well as atrio-ventricular and ventriculo-arterial relationships. Ventricular size and myocardial thickness is also well demonstrated. Magnetic resonance imaging thus allows for the characterisation of the connections between the large veins and the atria, the situs of the latter, and the connections between the ventricles and the large arteries, and it has a big advantage in visualisation of the great vessels outside the heart. This makes it possible to understand the complete flow pattern of the central circulation..

Magnetic resonance imaging of the most common congenital heart defects is often not performed if they can be fully understood by a combination of echocardiography and clinical findings. That is, however, if in the clinical findings are included the physiological assessment at an obviously invasive cardiac catheterisation with or without contrast injection (Table 2). The clinical use is dependent on age as magnetic resonance imaging normally is performed if needed after echocardiography in neonates and infants. Chest habitus and interposition of lungs constitute increasing echocardiographic problems when children grow older, or if there are postoperative changes. Thus magnetic resonance imaging is most helpful in older children.

The need for diagnostic catheterisation with its hazards can be minimised by using magnetic resonance imaging before the catheterisation. This is further also in line with the performance of interventional catheter techniques. The catheterisation should be performed only once for both diagnostic and therapeutic purposes which points at the need for precise diagnostic information in advance.

Magnetic resonance images the entire cardiovascular system. Thus it is especially useful in complex congenital heart malformations. Generally speaking it is more often indicated and provides more useful information the older the patient and the more complex the pathological anatomy is. Compared to catheterisation magnetic resonance imaging is completely non-invasive if sedation is not needed, it is without the hazards of ionising radiation and can be repeated many times for the above reasons. Thus it may be appropriate for the following anomalies in order of frequency in the general population.

Magnetic resonance techniques has thus often medium or high impact in the evaluation of congenital heart disease using a combination of T1 weighted spin echo sequences in different planes and faster gradient echo sequences. The first is used to image the anatomy of the heart and central vascular structures. The second is the used to image functional aspects of the cardiovascular system. The technique is non-invasive, uses no ionising radiation, and can be thus be used repeatedly. It is assessed that the fast technical development in the field of magnetic resonance techniques will lead to increasing usefulness in the evaluation of congenital heart disease.